In a general wireless communication system, at an edge part of a cell of a certain base station, the cell overlaps with that of a neighboring base station. In such an area where two or more cells overlap, interference may occur between radio waves from respective ones of the corresponding base stations. If such radio wave interference occurs, it results in deterioration in the quality of received signals, for example, SINR (Signal to Interference and Noise power Ratio). Accordingly, the certain base station comes to have no choice but to change the modulation method from that for high speed communication to that for medium to low speed communication. Therefore, a wireless communication system including a plurality of base stations has a problem in that the communication throughput may be reduced when a wireless terminal is present near a cell edge.
In this respect, in wireless communication systems such as of LTE-Advanced and WiMAX 2 (WiMAX is a trademark or registered trademark of the WiMAX Forum), “multi-base-station cooperative transmission technology” (may be referred to also as a distributed downlink coordinated multipoint framework or as coordinated multipoint transmission and reception) including such as a multi-base-station cooperative MIMO (Patent Literature 1) and a CoMP has been proposed. In the multi-base-station cooperative transmission technology, the same data is sent at the same timing from a plurality of base stations (the serving base station and its one or more neighboring base stations). A transmit diversity is thereby realized, and accordingly, the downlink throughput at the cell edges is improved.
In the above description, LTE is the abbreviation of Long Term Evolution. WiMAX is the abbreviation of Worldwide Interoperability for Microwave Access. MIMO is the abbreviation of Multiple Input Multiple Output. CoMP is the abbreviation of Coordinated Multiple Point.
Patent Literature 2 describes a base station which continuously manages the status of load such as the number of mobile terminals connected at each frequency. If a load condition is satisfied for one frequency and not satisfied for another frequency, the base station of Patent Literature 2 performs handover on the initiative of the base station, for load distribution.